Many electrical machines having electrical windings have a requirement for high reliability. While this requirement is widespread in many different types of electrical machines including motors, generators, solenoids, transformers and the like, in the case of motors or generators utilized in aircraft, the difficulty of repair in service as well as the need for unfailing operation in flight demands the highest quality and reliability of the motor or generator.
One way in meeting the requirements in aircraft is the known use of redundant systems and the provision of some means of shutting down a motor or generator if a problem occurs to prevent damage from occurring or at least to minimize the damage so that any needed repair is absolutely minimized or eliminated all together.
Typically, a problem will manifest itself by increased current flow within the machine which, in turn, results in increased heat generation. As a consequence, the temperature of the machine stator (as well as the rotor) will increase. This increase in temperature is conventionally sensed through the use of thermal protectors placed in the end turns of the stator windings of the machine. The thermal protectors may be in the form of fuses placed in series with the electrical circuit in which the dynamoelectric machine is located or they can be thermistors whose resistance changes with temperature and which are employed with a logic circuit to shut the dynamoelectric machine down when a certain temperature level is reached.
More common is the use of a small thermal switch located in the end turns of the stator winding. The switch is typically a very small bimetal switch and is configured so as to open or close when a certain temperature is reached. The opening or closing of the switch, in turn, opens or closes an electrical circuit which is then employed with other equipment to shut down the dynamoelectric machine before it overheats to the point of severe damage, and preferably before it overheats to the point of any damage whatsoever.
Upon shut down of the machine, the high temperature gradually dissipates and ultimately, the switch will revert back to its original condition, restoring the electrical circuit to its former state. This can be used as a means to automatically reset the motor shut down circuit and thus avoids any need to change fuses or the like to restore the motor to service.
To sense increasing temperature in an electrical machine, switches of the type mentioned are typically placed in the end turns of the windings of the machine stator. Because the switches universally include metal, they must be electrically isolated from the electrical circuit including the winding of the stator. This requirement, however, has led to system difficulties.
Specifically, if the insulation placed on the switch to isolate the switch from the stator windings is too thick, heat flow from the windings to the switch will be impeded. As a consequence, an undesirably high temperature may be reached in the windings and yet a substantial period of time may elapse before that temperature is sensed by the switch, giving rise to the opportunity for substantial electrical damage to the machine to occur in the interim.
Moreover, insulating the switches by common methods employed today complicates manufacture of the machines. Commonly used methods include wrapping the switch in various dielectric materials. See, for example, commonly assigned U.S. Pat. No. 4,571,518 issued Feb. 18, 1986 to Kintz et al. Another method employed is to "pot" the switch by molding a paste of potting compound around the switch. Both approaches require excessive manufacturing time due to the need to mix, handle, pot and cure the material. Further, either approach typically results in a relatively long thermal path from the end turns to the switch itself, frequently on the order of 20 to 30 thousandths of an inch or longer. Consequently, the time for the machine to tripout upon an overheated condition will be longer than desired and frequently can be excessive.
Still a further difficulty presents itself with the use of switches insulated by these methods. Because the methods result in a relatively large mass of insulation located about the switch, the size of the assembly to be inserted between end turns is larger than is desired. Consequently, individual wires in the end turns must be subjected to substantial force to be moved out of the way to create a pocket into which the switch can be inserted. During this process, the wires may be nicked or damaged by the tools used to form the pocket and this, in turn, has the potential effect of causing short circuits later when the motor is tested or placed in use. Consequently, a much smaller switch assembly mass is highly desirable.
Still another difficulty may attend the fabrication process. The switches that are used are very small. Typically, they may be shaped like a cylindrical disk and have a diameter of 3/10 of an inch and a thickness of only 1/10 of an inch. Purportedly, such switches are hermetically sealed at the source. However, long experience with such switches has caused many experienced manufacturing people to reach the opinion that a significant number of the switches may not remain hermetically sealed when subjected to a vacuum. Consequently, when the machine windings are exposed to vacuum during impregnation with varnish for insulation purposes, the varnish can conceivably infiltrate the switch if it is not hermetically sealed. When that occurs, the varnish within the switch may impede or even prevent the switch from changing its condition from open to closed, or vice versa, in response to temperature changes. When that occurs, the protection for the electrical machine that is desired becomes non-existent.
As a consequence, the manufacturing processes employed by the assignee of the present application include the assumption that the switch is not initially hermetically sealed, and further require that the insulating wrap around the switch provides a coating which will prevent varnish infiltration during the impregnation process. As a consequence, an expensive, multi-stage manufacturing process is required to assure that none of the previously stated circumstances will affect quality and performance of the electrical machines. Unfortunately, however, the use of the various steps deemed necessary is reflected in the cost of the machine to the customer.
The present invention is directed to overcoming the above problem.